The present invention relates to a laminated structure and a manufacturing method thereof, and in particular, to a laminated structure having a coating layer which is particularly suitable for an optical disk recording medium, and a manufacturing method thereof.
There are two processes for recording in a rewritable optical disk such as a CD-RAM or a DVD-RAM. One process is a photomagnetic recording and another process is a phase-changing recording. An alloy such as a TeFeCo alloy is used for a recording layer of the optical disk used for the photomagnetic recording. An alloy such as a GaSbTb alloy is used for a recording layer of the optical disk used for the phase-changing recording. Since the aforementioned alloys are easily oxidized, in general, a dielectric layer such as a silicon nitride layer is applied on the alloys by sputtering. The coating layer functions as a protective layer and prevents oxidization of the alloys. Further, characteristics such as chlorine resistance, moisture resistance, high refractive index, high light transmittance are required for the protective layer.
In general, a silicon nitride layer is formed by sputtering a silicon target material while nitride is introduced thereto. As an arc discharge easily occurs and it is difficult to apply high electric power to the silicon nitride layer, it is difficult to manufacture the silicon nitride layer. Further, there arise problems in that the silicon nitride layer has low light transmittance, refractive index and thermal conductivity.
In order to form the protective layer on the rewritable optical disk, in general, sputtering is used. One of the reasons why sputtering is used is that in order to reduce manufacturing costs, rather than a chemical method such as CVD method, a physical method such as sputtering, ion plating, vacuum disposition or the like is advantageous. Among the physical methods, in sputtering especially, a speed of forming the protective layer is high and the close-contact between the protective layer and a substrate is excellent. If a DC electric supply which enables high speed formation of the protective layer is used, a target material used in the sputtering must have a conductivity of 100 xcexa9xc2x7cm or less, preferably 10xe2x88x922xcexa9xc2x7cm or less in terms of a volume resistivity.
The present invention has been devised to solve the above-described problems and to provide a laminated structure which is suitable for an optical disk recording medium such as a CD-RW, a DVD-RAM or the like by providing a silicon carbide coating layer which is excellent in chlorine resistance, humidity resistance, and has high refractive index and high light transmittance, and a manufacturing method thereof which enables easy and reliable manufacturing of the laminated structure.
As a result of their diligent study in order to attain the aforementioned object, the present inventors noted the following points. That is, as a result of research to obtain alternatives to the target material necessary for preparing a derivative such as a silicon nitride, as a target material used in sputtering, it was found that most metals generally have large gravity and thus, it is difficult to handle them. Further, in order to obtain a light reflectance necessary for the protective layer in order to form a pure metallic thin layer which has the same property as the target material at the time of sputtering, the thickness of the protective layer must be controlled. Therefore, manufacturing of such a target material is difficult. Alternatively, most ceramics materials are usually insulating materials. Thus, the ceramics materials are not suitable for the target material. A sintered silicon carbide is effective as the target material which can control the optical characteristics of the protective layer which can be formed thin depending on sputtering conditions.
As the target material which can control the optical characteristics of the protective layer depending on the sputtering conditions, a sintered silicon carbide to be described below is effective. The sintered silicon carbide has high density, high purity and a volume resistivity of 100xcexa9xc2x7cm or less, and is obtained by carrying out a step of charging a nitrogen-containing compound at the time of preparing a mixture of silicon carbide powder and non-metallic sintered aid or a step of charging the nitrogen-containing compound at the time of mixing carbide material and silicon material which are materials for silicon carbide powder during manufacturing thereof, in a manufacturing process of a sintered silicon carbide disclosed in Japanese Patent Application Laid-Open (JP-A) No. 10-67565 which has already been proposed by the present inventors. Further, a sintered silicon carbide is more effective, which has an impurity ratio of 1.0xc3x971011 atoms/cm2 or less on or in a vicinity of its surface due to a washing method disclosed in Japanese Patent Application Nos. 10-348569 and 10-348701 being carried out, and which has high density, high purity and a volume resistivity of 100 xcexa9xc2x7cm or less.
The present invention is based on the above-described findings of the present inventors, and includes the following aspects.
 less than 1 greater than  A laminated structure having a silicon carbide coating layer formed on an alloy substrate by sputtering, wherein the silicon carbide coating layer has a light transmittance of 70% or greater.
 less than 2 greater than  The laminated structure according to less than 1 greater than , wherein the alloy substrate is formed of a magnetic alloy or a phase-changing alloy.
 less than 3 greater than  The laminated structure according to less than 1 greater than  or less than 2 greater than , wherein an impurity ratio on the surface of the silicon carbide coating layer is 1.0xc3x971012 atoms/cm2 or less.
 less than 4 greater than  The laminated structure according any of less than 1 greater than  through less than 3 greater than , wherein the thickness of the silicon carbide coating layer is 10 to 100 nm.
 less than 5 greater than  A manufacturing method of a laminated structure having a silicon carbide coating layer formed by sputtering with a target material, wherein the silicon carbide coating layer is formed by controlling electric power inputted to a sputtering device, flow rate of oxygen gas or nitrogen gas introduced and sputtering time.
 less than 6 greater than  The manufacturing method of a laminated structure according to less than 5 greater than , wherein the target material is a sintered silicon carbide, and the sintered silicon carbide has an impurity ratio of 1.0xc3x971011 atoms/cm2 or less on and in a vicinity of the surface thereof, a density of 2.9 g/cm3 or greater, and a volume resistivity of 100 xcexa9xc2x7cm or less.